1. Field of the Invention
This invention relates to FET constant current sources and, more particularly, to temperature compensation in such circuits.
2. Discussion of the Related Art
Integrated circuits (ICs) often require a constant current source; that is, a current reference that is both accurate and stable with respect to temperature and variations in manufacturing process. In the prior art, the ICs that implement such a constant current source are typically both complex and inefficient; that is, wasteful in terms of chip area utilized and power consumed. Constant current sources that are illustrative of prior art approaches include D. A. Badillo, IEEE Symp. on Circuits and Systems-III, Vol. 3, pp. 197–200 (May 2002) and R. Dehghani et al., IEEE Symp. on Circuits and Systems-II, Vol. 50, No. 12, pp. 928–932 (December 2003), both of which are incorporated herein by reference. The Badillo paper describes a CMOS current reference circuit that places a level shift stage between a feedback amplifier and a bandgap reference (BGR) voltage source in order to increase the temperature operating range. The Dehghani et al. paper also describes a CMOS current reference circuit based on a BGR voltage source and a CMOS circuit similar to a beta amplifier but modified by the inclusion of an NMOS transistor that functions as a resistor. The NMOS transistor is operated in the triode region to achieve a current that has a negative temperature coefficient and only oxide thickness dependence. The BGR voltage has a positive temperature coefficient that cancels the negative temperature coefficient of the beta amplifier.
Thus, a need remains in the art for an accurate, stable constant current source, which can be implemented in MOS technology without the complexity and inefficiency typified by prior art designs.